Chametz (חָּמֵץ) refers to leavened products derived from any of the five grains — wheat, barley, rye, oats, spelt — that have come into contact with water for longer than eighteen minutes. During the seven days of Passover (eight in the diaspora), the consumption, possession, and benefit from chametz is prohibited.
The chametz rule has a direct consequence for spirits. Most gins, whiskies, and vodkas are distilled from a grain mash; the mash is fermented, which qualifies as chametz processing under the strictest interpretations. Even though distillation arguably purifies the spirit beyond its grain origin, the leading authorities treat grain-distilled spirits as chametz for Passover purposes.
Specialist kosher-for-Passover spirits exist, distilled from cane sugar, grape, or potato bases. These are a distinct, stricter certification category from year-round kosher spirits.